The present invention relates to micropositioning devices in general and more particularly to micropositioning devices intended for aligning optical components.
Conventional translation devices with one degree of freedom comprise a mobile platform sliding on a base frame. The range of movement determined by this sliding, or kinematic, guide is controlled by an actuator, whose body is fixed to the base frame and whose mobile part is connected to the platform by an appropriate transmission means. When the movement of the actuator is approximately rectilinear and parallel to the translational axis of the guide, the transmission means is often no more than the platform being supported against the actuator through the intermediary of a ball. This support is ensured by a spring stretched between the frame and the platform.
The best known actuators are of two types: those with micrometer screws or those with piezoelectric disk stacks.
To achieve translation with two degrees of freedom, the frame of a second similar device is fixed to the mobile platform of a first device with one degree of freedom, the second device being orientated, however, in a different direction, for example at a right angle to the first device. FIG. 1 shows a conventional translation device with two degrees of freedom. Similarly, by fixing three one-dimensional modules in series a device with three degrees of freedom is obtained.
Conventional kinematic guiding devices allow the maintenance of a high degree of precision, typically between one and ten microns, for movements of relatively large range, in the order of 10 to 100 mm. Large movements are controlled by screw actuators. Sometimes a piezoelectric actuator is mounted in series with the screw, between the bearing ball and the mobile platform. This arrangement theoretically allows very great sensitivity, corresponding to a resolution lower than a 100th of a micron. However, such performances are illusory if they are not accompanied by corresponding stability and reproducibility on the part of the guide. The main limitations of kinematic systems are caused by the machining precision of the slideways, play, friction and wear.
In general, these devices remain ill-adapted to certain uses requiring a typical precision of a 10th of a micron, as, for example, the positioning or mounting of monomode guided optics components.
In cases where displacement range is low, typically less than or equal to one mm., it has been proposed to utilize the principles of the elastic guide, as described for example in the article by P. H. Sydenham, entitled "Elastic Design of Fine Mechanism in Industry" and published in J. Phys. E. Sci. Instruments, Vol. 17 (1984) p. 922.
In this type of guide, use is made of the ease with which a thin and elongate strip, fixed at one end and acted upon at the other end, lends itself to deflection in a direction perpendicular to the strip, while remaining rigid in the parallel directions. This deflection of the strip is equivalent to a rotation of its free end around an axis situated in its plane when it is at rest. By an adequate combination of four such elastic "pivots", a translation according to the diagram of FIG. 7 can be obtained. This parallelogram configuration can be further improved so as to render the translation rectilinear, by means of a second parallelogram. This latter structure, shown in FIG. 8, can be used to effect translational guidance with a rectilinear coordinate.
When the deformations are calculated so as to remain well short of the elastic limits and the joints are cut in the body to prevent the effects of rigid fixing, a device is obtained without play, friction or wear, which is perfectly accurate and reproducible. Precision is limited only by the effect, combined with the finite rigidity of the strips in the directions perpendicular to the planned displacement, of a variable load applied in these directions.
A monolithic elastic system can be advantageously effected by the known method of machining by electroerosion. A complex, monolithic, elastic guidance device, controlled by piezoelectric actuators, is reported by R. D. Young, in his article entitled "Moving Stage Improves Accuracy by Microcircuit Measuring Technique" published in Research & Development (April 1984), p. 114.
However, the implementation of micropositioning devices with elastic joints comes up against various problems connected with the integration of actuators into the system. Although piezoelectric actuators provide extremely high resolution, they have a number of disadvantages, including: the need for a control voltage which can reach values higher than 1,000 volts, non-linear response and the existence of high hysteresis. The use of such actuators necessitates the presence of a displacement detector incorporated in a control loop connected to each actuator.
Micrometer screw actuators have the advantage of large displacement but the disadvantage of being relatively heavy and bulky. With elastic guide systems, which are sensitive to the effects of load and vibrations, it is impossible to arrange in series several positioning modules with one degree of freedom in order to achieve displacement according to several coordinates if it is desired to take advantage of their properties and their high intrinsic precision.